Program Evaluation and Review

Technique (PERT)

PERT

• Bar charts and CPM networks assume all activity durations

are constant or deterministic.

• The assumption of constant durations may not be realistic

because of working conditions.

• To address the variability of project activities, the Program

Evaluation and Review Technique (PERT) was developed.

• PERT incorporates uncertainty into the project by assuming

that the activity durations of some or all of the project

activities are variable.

PERT

• Three time estimates are required to compute the parameters of an activity’s duration distribution:

• pessimistic time (tb ) - the time the activity would take if things did not go well

• most likely time (tm ) - the consensus best estimate of the activity’s duration

• optimistic time (ta ) - the time the activity would take if things did go well

Mean (expected time): te =ta + 4 tm + tb

6

Variance: Vt =δ2 =tb - ta

6

2

Where,

tb is the expected duration

Tb is the most optimistic duration estimate

tm is the most likely duration estimate

Ta is the most pessimistic duration estimate

PERT Chart may look like

Draw the network

Use PERT formula to calculate the expected duration for each

activity of the project

Analyze the paths through the network and find the critical path

The length of the critical path is the mean of the project

duration probability distribution, It follows a normal distribution

The standard deviation of the project duration probability

distribution is computed by adding the variances of the critical

activities (all of the activities that make up the critical path) and

taking the square root of that sum

Probability computations can now be made using the normal

distribution

PERT ANALYSIS

Probability computation

Determine probability that project is completed within

specified time

Z = - N

where = tp = project mean time

= project standard mean time

N = (proposed ) specified time

Normal Distribution of Project Time

= tp Timex

Z

Probability

PERT Example

Activity tm to tb Mean Variance

1 3 1 5 3 0.4

2 6 3 9 6 1.0

3 13 10 19 13.5 2.3

4 9 3 12 8.5 2.3

5 3 1 8 3.5 1.4

6 9 8 16 10 1.8

7 7 4 13 7.5 2.3

8 6 3 9 6.0 1.0

9 3 1 8 3.5 1.4

Path 1 = Activity 1,4,8

Mean = 17.5 Variance=3.7

Path 2 = Activity 1,5,6

Mean = 16.5 Variance=3.6

Path 3 = Activity 1,5,7,9

Mean = 17.5 Variance=5.4

Path 4 = Activity 2,6

Mean = 16.0 Variance=2.8

Path 5 = Activity 3,9

Mean = 17.0 Variance=3.6

What is the probability that the project can be completed in N days?

Z = - N

Benefits of CPM/PERT

• Useful at many stages of project management

• Mathematically simple

• Give critical path and slack time

• Provide project documentation

• Useful in monitoring costs

•How long will the entire project take to be completed? What are

the risks involved?

•Which are the critical activities or tasks in the project which could

delay the entire project if they were not completed on time?

•Is the project on schedule, behind schedule or ahead of

schedule?

•If the project has to be finished earlier than planned, what is the

best way to do this at the least cost?

CPM/PERT can answer the following important questions:

Limitations to PERT

• PERT assumes statistically independence between

activities. This may not be appropriate

• PERT consistently underestimates the expected project

completion time due to alternate paths becoming

critical. The principal cause of this underestimation is a

condition known as “merge event bias”

• PERT results are too optimistic

To overcome the limitation, Monte Carlo simulations can

be performed on the network to eliminate the optimistic

bias

GANTT CHART vs PERT CHART

GANTT CHART vs PERT CHART

GANTT chart visually shows the duration of Tasks whereas a PERT

chart visually shows the sequence dependencies between tasks.

GANTT visually shows the Time overlap of Tasks whereas a

Network does not show time overlap but does show which tasks

could be done in parallel.

Some form of GANTT chart can visually show Slack Time available

within an Earliest Start and Latest finish time.

Most Project Managers find PERT very helpful for scheduling,

monitoring and controlling Projects.

PERT is recommended for Large Projects with high inter-task

dependencies and the GANTT chart for simpler Projects.

Most Project Management Case Tools nowadays (eg. MS-Project,

Primavera ) allow the best feature of PERT to be incorporated into

GANTT Charts.

Line of Balance Method

Line of Balance Method (LOB)

A graphical method for production control integrating bar chart and

production curve concepts.

This is most suited and efficient for linear constructions and repetitive

activities

LOB focuses on the planned versus actual progress for individual activities

and provides a visual display depicting their difference.

LOB enables management to provide accurate control by prioritizing

reallocation of resources. This assumes resources are interchangeable

Linear Construction Operations

Often construction sites have linear properties that influence the

production sequence.

A road job may be subdivided into many sections.

A set of work processes need to be completed in a particular sequence

before a section is completed:

(1) rough grading, (2) finish grading, (3) aggregate base installation, (4)

5-in.concrete pavement, (5) 9-in. concrete pavement, and (6) curb

installation.

Linear Construction Operations

Each of the 14 sections can be thought of as being processed by crews

and equipment representing each of the six work processes.

Work is proceeded from section 1, section 2, …, to section 14. The six

work processes are carried out in sequence.

There are work activity overlaps such that several operations are in

progress simultaneously during the middle of the job.

Many types of projects exhibit a kind of rigid work sequence and have

a “train” effect.

High-rise building

Each floor has to pass a set of operations: erect formwork, install

reinforcing steel and pure concrete.

Tunnel

Drill, blast, remove muck, and advance drilling shield..

Elements of LOB

There are four Elements of

LOB

1. The objective chart

2. The program chart

3. The progress chart

4. The comparison

Objective chart

A segmental curve showing

cumulative end products to

be produced over a calendar

time period.

Objective chart

Program Chart

It is a flow process chart of all major activities, illustrating their planned,

sequenced interrelationships on a “lead-time” basis.

Three Aspects to Consider in Developing Program Chart

(1) Operations to be performed

(2) Sequence of operations

(3) Processing and assembly lead time

Progress Chart

Progress chart indicates the cumulative progress or status of actual

performance at each monitoring point.

Comparison of Actual Progress and Expected Progress

LOB Applied to Construction

A road project has 40 same sections and each section has 8

activities. Suppose they are balance production. Each week has 40

working hours. The production curve and the lead time are shown as

follows. Calculate the LOB values (completed sections) of each

control points at the beginning of fifth week.

8hr8hr2hr16hr6hr4hr14hr2hr

9

60

8

58

7

44

6

40

5

34

4

18

3

16

2

8

1

0

A B C D E F G H

40

30

20

10

0

Sections

12

28

40 80

21

120

3

160

4

200

5

240

6

280

7

Hours

Weeks0

0

Lead time

Production curve

Section

40

30

20

10

0ActivityHGFEDCBA

Answer:

LOB(1)= 0.1×3×40+0.2×1×40 = 20 sections

LOB(2)= 20+0.2×8 = 21.6 sections

LOB(3)= 21.6+0.2×8 = 23.2 sections

LOB(4)= 23.2+0.2×2 = 23.6 sections

LOB(5)= 23.6+0.2×16 = 26.8 sections

LOB(6)= 26.8+0.2×6 = 28 sections

LOB(7)= 28+0.15×4 = 28.6 sections

LOB(8)= 28.6+0.15×14 = 30.7 sections

LOB(9)= 30.7+0.15×2 = 31 sections

A B C D E F G H0

10

20

30

40

Activity

Planned Progress Chart

Comparative Progress Chart

Scheduling with Computer Software

Introduction to Computer software

Project planning and scheduling are key factors to completing a

construction project within budget, on time and with few problems

The most efficient way to develop a construction schedule is to use

computer programs

Computer programs also proved useful in resource allocation.

The lesson learned from previous projects could also be stored in

historical files. This improved future planning as well as estimating

process

A wide variety of affordable project management software packages

are available for purchase. These packages allow the project

manager and project team to plan and control projects in a

completely interactive mode.

Common features of project management software

Create lists of tasks with their estimated durations

Establish interdependencies among tasks

Work with a variety of time scales, including hours, days, weeks, months, and

years

Handle certain constrains

Track team members, including their pay rates, hours worked thus far on project

and upcoming vacation dates

Incorporate company holidays, weekends, and team members vacation days into

calendaring systems

Handle shifts of workers (day, evening, night)

Monitor and forecast budgets

Common features of project management software

Look for conflicts; for example, resource over allocated and time conflicts

Generate a wide variety of reports

Interface with other software packages such as spread sheets and database

Sort information in a variety of ways; for example, by project, by team member, or

by work package

Handle multiple projects

Work on-line and respond quickly to changes in schedule, budget, or personnel

Compare actual costs with budgeted costs

Display data in a variety of ways, including both Gantt charts and network

diagrams

Computer Software for Project Management

Various computer programs have been established so far with regard to preparing

schedules:

Microsoft Project (Microsoft Corp.)

Mac Project (Claris Corp.)

Power Project (ASTA Development Inc.)

Primavera Project Planner (Primavera)

Project Scheduler (Scitor Corp.)

Project Workbench (ABT Corp.)

However, out of which Microsoft project and Primavera Project Planner are two

popular software programs used in construction industry.

Benefits of using project management software

Organizing the plan and thinking through the details of what must be done

Scheduling deadlines that must be met

Scheduling the tasks in the appropriate sequence

Assigning resources and costs to tasks and scheduling tasks around

resource availability

Fine-turning the plan to satisfy time and budget constraint or to

accommodate changes

Preparing professional looking reports to explain the project to owners,

top management, supervisors, workers, subcontractors and the public.

Limitations of using project management software

Although scheduling software packages have supported resource-loading

activities for sometime, they still require manual input of resource

information and do not represent the reasons behind the selection of a

particular crew and crew size

In most cases, computer programs give adequate solutions; however,

when resource requirements exceed the resources available, the

computer programs do not provide the optimum scheduling solutions.

Activity Codes Calendars

Screen Layout of Project SNP1 in Primavera Project

Planner 3.1

Activity Bar

Activity ID

Activity Description

Activity codes

Relationship between Activities

Bar Chart

Data for Each Activity in The Project

Table of Resources Needed in The Project

Layout for Project Reporting

PERT or PDM View

Graph of Resources which Needed in This Project

Screen Layout of MS Project

Relationship between Activities

Assigning Resources Needed in The Project

Layout of network diagram